1. Field of the Invention
This invention relates to a circuit for forming an electric signal which is proportional to a flux component of a rotating field machine. A voltage proportional to the Y-voltage belonging to the flux component is fed to an integrator. The voltage signal proportional to the flux component is taken off at the output, and a zero-controller is provided for suppressing the DC component. The input of the zero-controller is connected to the output of the integrator and the output of the zero-controller is connected to a summing point at the input of the integrator.
2. Description of the Prior Art
Such a circuit arrangement is known from German Auslegeschrift No. 26 35 965. The term "rotating-field machines" includes synchronous and asynchronous machines which can be operated as motors or generators.
The prior art circuit arrangement forms the actual flux value by integration of the terminal voltage of the rotating-field machine, and the current is utilized to take the ohmic stator voltage drops and reactive (inductive stray) voltage drops into consideration. Two such circuit arrangements for two phases of the rotating-field machine are required in a three-phase rotating-field machine. The two flux components so established determine the position of the flux vector and its magnitude. Information regarding the position and magnitude of the flux vector makes it possible to operate the rotating-field machine with field orientation (see Siemens-Zeitschrift 1971, pages 765 to 768 and German Pat. No. 23 53 594).
To avoid drifting of the integrator due to DC components, a PI zero-controller is employed in the prior art circuit. The choice of the parameters of the PI zero-controller controls determines the amplitude and phase error of the flux component determined. In this case, the phase error depends on the speed. When the PI zero-controller is designed so that the phase error remains sufficiently small at the lowest operating frequency of the rotating-field machine, very high gains occur when there are beats between the machine and the network frequency, so that instability can occur when drive control actions are performed.
It is particularly important to measure the flux components as accurately as possible when the rotating-field machine is being started, since a measuring error, in the worst case, can lead to the inability of the rotating-field machine to start. In the known circuit arrangement, the zero-control is disconnected for this purpose. In this connection, the determination of the instant when the zero-controller is switched on and when it is to be switched on while the machine is running is a problem, since transients can occur in this process and lead to instability.
It is an object of the invention to develop a circuit of the type mentioned above in which the flux component can be determined with its true amplitude, with a small and constant phase error, independently of the frequency of the rotating-field machine, and in which synchronized switching on of the zero-controller becomes unnecessary.